A semiconductor package is a metal, plastic, glass, or ceramic casing containing one or more semiconductor electronic components typically referred to as integrated circuit (IC) die. Individual discrete IC components are formed using known semiconductor fabrication techniques (e.g., CMOS) on silicon wafers, the wafers are then cut (diced) to form individual IC die, and then the IC die are the assembled in a package (e.g., mounted on a package base substrate). The package provides protection against impact and corrosion, holds the contact pins or leads which are used to connect from external circuits to the device, and dissipates heat produced in the IC die.
Flip-chip packages are a type of semiconductor package in which two structures (e.g., an IC die and a package base substrate) are stacked face-to-face with interconnect structures (e.g., solder bumps or pins) disposed in an intervening gap to provide electrical connections between contact pads respectively formed on the two structures. The gap between the two structures ranges from microns to millimeters.
A micro-spring package is specific type of flip-chip semiconductor package in which electrical connections between the IC die and the package base substrate are provided by way of tiny curved spring metal fingers known as “micro-springs”. Micro-springs are batch-fabricated on a host substrate (i.e., either the IC die or the package base substrate), for example, using stress-engineered thin films that are sputter-deposited with a built-in stress gradient, and then patterned to form individual flat micro-spring structures having narrow finger-like portions extending from associated base (anchor) portions. The narrow finger-like portions are then released from the host substrate (the anchor portion remains attached to the substrate), whereby the built-in stress causes the finger-like portions to bend (curl) out of the substrate plane with a designed radius of curvature, whereby the tip end of the resulting curved micro-spring is held away from the host substrate. The micro-spring package utilizes this structure to make contact between the host substrate (e.g., the IC die) and a corresponding package structure (e.g., the package base substrate) by mounting the IC die such that the tip ends of the micro-springs contact corresponding contact pads disposed on the corresponding package structure.
For high performance and high power IC's such as microprocessors, metal blocks combining with a bulky fan are attached directly to the backside (i.e., non-active surface) of chips disposed in a flip-chip arrangement for cooling purposes. Most of the heat (˜80-90%) is conducted across the bulk of the chip, and then metal block, and finally dissipated through force convection by the fan. If avoid sticking a bulky fan on chip's back, the heat dissipation path needs to be engineered.
Driven by the trend of thinner, lighter and more and more functions in electronics products like cell phones and TVs, higher power density in semiconductor packaged devices is an unavoidable trend. Therefore, there is a need to manage the heat generated in the package in a more efficient and controllable way. Bulky fan is no longer an efficient way to manage the heat, especially the chips tends to be stacked horizontally as well as vertically (3D stacking). Passive methods like heat spread, underfill, and thermal interface materials, all of them are hard to be applied to chip stacking applications. Active cooling like micro fluidic channels can be used for 3D stacking, but fluid is not common in consumer electronics.
Ionic wind (or ion wind) is a dry process that may be used for IC cooling. Ionic wind works by applying high voltage between a high curvature (emitting) and a low curvature (collecting) electrodes. High electrical field around the emitting electrode ionizes the air molecules. The ions accelerated by electrical field and then transfer momentum to neutral air molecules through collisions. The resulting micro-scale ionic winds can potentially enhance the bulk cooling of forced convection at the location of a hot spot for more effective and efficient cooling. Various approaches have been developed that have been shown to generate ionic wind using, for example, wire based corona discharge. However, these approaches are difficult to implement using existing high volume IC fabrication and production methods.
What is needed is a practical, low cost method for generating ionic wind that can be implemented between circuit structures (e.g., a base substrate and an IC die) in a semiconductor circuit assembly (e.g., a flip-chip package) to cool the circuit structures.